1. Field of the Invention
The present invention relates to an apparatus for measuring the upper surface height and/or thickness of a work, such as a semiconductor wafer, held by a chuck table provided in a machining apparatus such as a laser beam machining apparatus, and to a laser beam machining apparatus equipped with the measuring apparatus.
2. Description of the Related Art
In a semiconductor device manufacturing process, a plurality of regions are demarcated in a surface of a substantially circular disk-like semiconductor wafer by planned dividing lines called streets which are arranged in a lattice pattern, and devices such as ICs and LSIs are formed in the demarcated regions. Then, the semiconductor wafer is cut (diced) along the streets to divide the regions with the devices formed therein, whereby individual semiconductor chips are manufactured. As a method for dividing the wafer such as a semiconductor wafer and an optical device wafer along the streets, there has been attempted a laser beam machining method in which a pulsed laser beam capable of being transmitted through the wafer is used, and the wafer is irradiated with the laser beam, which is converged to a point in the inside of the regions to be divided. In the dividing method based on the laser beam machining method, specifically, a laser beam having a wavelength of, for example, 1064 nm which can be transmitted through the wafer is used to irradiate the wafer therewith from one side of the wafer while setting the converging point of the laser beam in the inside of the wafer, to continuously form a denatured layer inside the wafer along the streets, and an external force is exerted along the planned dividing lines where the wafer is lowered in strength due to the denatured layer formed there, whereby the work is divided (refer to, for example, Japanese Patent No. 3408805).
In addition, as a method for dividing a plate-shaped work such as a semiconductor wafer, there has been proposed a method in which irradiation with a pulsed laser beam is conducted along the streets in the wafer so as to form laser beam-machined grooves, and the wafer is cut (diced) along the laser beam-machined grooves by a mechanical breaking apparatus (refer to, for example, Japanese Patent Laid-open No. Hei 10-305420). In the case of thus forming the laser beam-machined grooves along the streets formed in the work, also, it is important to position the converging point of the laser beam at a predetermined height with respect to the work.
Besides, as a machining method by which a hole (via hole) reaching an electrode called a bonding pad is formed in a surface of a semiconductor wafer at the position of the bonding pad from the back side of the wafer, there has been attempted a method in which the semiconductor wafer is irradiated with a pulsed laser beam from the back side thereof. However, when the semiconductor wafer has a dispersion of thickness, it may be impossible to accurately form the hole (via hole) reaching the bonding pad. Therefore, it may be necessary to accurately grasp the thickness in the bonding pad area of the semiconductor wafer.
However, a plate-shaped work such as a semiconductor wafer often has undulation and has a dispersion of the thickness thereof, so that it is difficult to achieve uniform laser beam machining of such a work. More specifically, in the case of forming a denatured layer in the wafer along the streets, the presence of a dispersion of wafer thickness would make it very difficult to uniformly form the denatured layer at a predetermined depth position, due to the relationship thereof with the refractive index, in irradiating the wafer with the laser beam. Also, in the case of forming the laser beam-machined grooves along the streets formed in a wafer, the presence of a dispersion of wafer thickness would make it very difficult to form the laser beam-machined grooves with a uniform depth. Furthermore, in the case of forming a hole (via hole) reaching a bonding pad in a wafer, the presence of a dispersion of wafer thickness would make it very difficult to accurately form the hole (via hole) reaching the bonding pad.